This invention relates to compositions comprising metallic particles and solid oxidizer, and a method for preparing such compositions. The compositions comprise a homogeneous mixture of predetermined stoichiometry, suitable for use in a solid rocket propellant, of metallic particles and solid oxidizer wherein individual metallic particles are generally uniformly distributed throughout a solid oxidizer matrix.
Solid rocket motor propellants are widely used in a variety of aerospace applications, such as launch vehicles for satellites and spacecraft. Solid propellants have many advantages over liquid propellants for these applications because of their good performance characteristics, ease of formulation, ease and safety of use, and the simplicity of design of the solid fueled rocket motor when compared to the liquid fueled rocket motor.
The conventional solid propellant typically consists of an organic or inorganic solid oxidizing agent, a solid metallic fuel, a liquid polymeric binder, and a curing agent for the binder. Additional components for improving the properties of the propellant, i.e., processability, curability, mechanical strength, stability, and burning characteristics, may also be present. These additives may include bonding agents, plasticizers, cure catalysts, burn rate catalysts, and other similar materials. The solid propellant is typically prepared by mechanical mixing of the oxidizer and metallic fuel particles, followed by addition of the binder and curing agent with additional mixing. The resulting mixture is then poured or vacuum cast into the motor casing and cured to a solid mass.
The solid propellant formulations most widely used today in such applications as the Space Shuttle solid rocket booster and Delta rockets contain as key ingredients aluminum (Al) particles as the metal fuel and ammonium perchlorate (AP) particles as the oxidizer. The Al and AP particles are held together by a binder, which is also a fuel, albeit one of substantially less energetic content than the metal. The most commonly used binder comprises hydroxy-terminated polybutadiene (HTPB). This particular type of propellant formulation is favored for its ease of manufacture and handling, good performance characteristics, reliability and cost-effectiveness.
A typical Al+AP solid rocket propellant formulation consists of 68 wt. % AP (trimodal particle size distribution, i.e., 24 wt. % 200 xcexcm, 17 wt. % 20 xcexcm, 27 wt. % 3 xcexcm), 19 wt. % Al (30 xcexcm average particle diameter), 12 wt. % binder (HTPB) and isophorone diisocyanate (IPDI) curing agent), and 1 wt. % burn rate catalyst (e.g., Fe2O3 powder).
The relative amounts of the components in this formulation are chemically stoichiometric. In other words, there should be just enough oxidizer molecules present in the formulation to completely react with all the fuel molecules that are present, with no excess of either oxidizer or fuel. This formulation contains one oxidizer (AP) and two distinct fuels, i.e., Al and binder. The weight ratio of AP to Al for a stoichiometric mixture, i.e., no excess oxidizer or fuel, is 42:19. The weight ratio of ammonium perchlorate to binder for a stoichiometric mixture is 26:12. These ratios are the same regardless of any other components that may be present in the mixture.
Because of their burn characteristics, conventional Al/AP propellants are most suitable for use in conjunction with a particular motor design. This design is the hollow core or center perforated (CP) core motor design in which the propellant grain is formed with its outer surface bonded to the inside of the rocket motor""s casing with a hollow core extending through most or all of the length of the grain. The burning front progresses outwardly from the core to the case. This motor design is bar far the most common design for solid fuel motors. One example of a current application utilizing this design is the Space Shuttle, which uses solid motors which are 150 ft. long and 12 ft. in diameter with a 4 ft. hollow core.
The propellant grain in a CP design must have substantial structural integrity to keep the grain intact during operation. A binder is therefore used to xe2x80x9cgluexe2x80x9d the particulate components of the propellant together. During the initial mixing of the propellant, the percentage of the binder, initially in the form of a liquid resin, is high enough to maintain a relatively low viscosity, such that the propellant is in a slurry form, allowing the propellant mixture to be poured or injected into the motor casing. A mandrel is placed in the middle of the motor casing to create the hollow core (typically before the propellant is poured into the core) and is removed once the propellant has cured.
Propellants comprising a metal fuel in combination with a solid oxidizer may be used in other applications outside of aerospace, including gas generators. Solid propellants are also used in launch vehicles, e.g., NASA rockets, Space Shuttle, French Ariane rockets. Virtually all launch vehicles use a combination of liquid fuel motors with solid fuel boosters. Both the Delta III and the Space Shuttle are examples having combined liquid and solid motors. The Delta rocket has a main liquid motor with nine smaller strap-on solid boosters, while the shuttle has three onboard liquid motors with two strap-on solid boosters.
Although enormous innovations have occurred in guidance, electronics and virtually every part of spacecraft to date, the propulsion systems have remained essentially the same for decades. Boeing""s Delta III, introduced in 1998, utilizes a liquid engine that was designed in the 1960""s and is fueled by kerosene and oxidized by liquid oxygen. The solid boosters were designed in 1961 and are virtually unchanged since then, except for an epoxy motor casing. Additionally, over the past decade, almost every system on the Shuttle has been replaced or upgraded, except for its propellant. It is therefore desirable to provide a novel solid rocket propellant that affords superior performance to the conventional propellants in current use today.
In accordance with this invention, it has been found that a composition comprising a substantially homogeneous mixture of a metallic substance and a solid oxidizer can be prepared, wherein the metallic substance is generally uniformly distributed in the form of discrete particles throughout a matrix of solid oxidizer. Other embodiments include utilizing aluminum as the metallic substance, ammonium perchlorate as the solid oxidizer, and metallic particles less than about one micron in diameter. In a preferred embodiment the metallic substance and oxidizer are present in stoichiometric amounts.
In one embodiment of this invention, the mixture of metallic particles and solid oxidizer is prepared by a method that involves dissolving the oxidizer in a suitable solvent, mixing the metallic particles with the solution while agitating the solution to ensure a generally uniform dispersion, removing the solvent, and recovering a powder comprising individual metallic particles generally uniformly distributed throughout a solid oxidizer matrix. The solvent is removed so as to preserve the relative distribution of the metallic particles in the oxidizer matrix.
In one embodiment of this invention, freeze drying used to remove the solvent. The solution containing the metallic particles is rapidly frozen, then the solvent is removed by sublimation under vacuum conditions. Another embodiment utilizes spray drying to remove the solvent.
A further embodiment of this invention is to provide a composition wherein the solid oxidizer and the portion of the metallic substance capable of undergoing an oxidation reaction are present in substantially stoichiometric amounts.
A further embodiment of this invention is to provide a composition comprising a homogeneous mixture of metallic particles and solid oxidizer of a pre-selected stoichiometry, wherein the metallic particles comprise aluminum particles less than about one micron in diameter.
A further embodiment of this invention is to provide a composition comprising substantially discrete matrices of substantially similar stoichiometry, each matrix comprising solid oxidizer having a pluriality of particles of a metallic fuel embedded therein wherein the average distance separting the metallic fuel particles is controlled.
In a further embodiment there is provided a composition comprising substantially discrete particles of substantially similar stoichiometry, each particle comprising a solid oxidizer matrix substantially encapsulating a plurality of particles of a metallic substance.
In a further embodiment of the invention there is provided a composition comprising an intimate, stoichiometric mixture of an oxidizer and metallic fuel particles.
Introduction
The following description and examples illustrate preferred embodiments of the present invention in detail. Those of skill in the art will recognize that there are numerous variations and modifications of this invention that are encompassed by its scope. Accordingly, the description of preferred embodiments should not be deemed to limit the scope of the present invention.
As used within this specification, the term xe2x80x9cstoichiometricxe2x80x9d refers to a mixture of chemical components having the exact proportions required for complete chemical combination or reaction. In terms of a rocket fuel composition, a stoichiometric mixture is one in which the components involved in the combustion process, including the metallic fuel and oxidizer, are present in exactly the quantities needed for reaction, without an excess of any component left over after the reaction.
The term xe2x80x9cstoichiometryxe2x80x9d refers to the ratio of oxidizer to fuel components in a mixture. The stoichiometry, or ratio, may be xe2x80x9cstoichiometricxe2x80x9d, i.e., the oxidizer and fuel components are present in such amounts so that complete combustion occurs without any excess oxidizer or fuel. The stoichiometry may also be xe2x80x9cnon-stoichiometricxe2x80x9d, i.e., excess oxidizer or fuel is present in the mixture over that which is required for complete combustion of the mixture.
The term xe2x80x9chomogeneousxe2x80x9d refers to a mixture or blend of components that is generally uniform in structure and composition with little variability throughout the mixture. Different portions of a homogeneous mixture exhibit essentially the same physical and chemical properties substantially at every place throughout the mixture. The stoichiometry in a homogeneous mixture is also substantially constant throughout the mixture.
The term xe2x80x9cmetalxe2x80x9d refers to alkali metals, alkaline earth metals, rare earth metals, transition metals, as well as to the metalloids or semimetals.
The term xe2x80x9cmetallicxe2x80x9d refers to any substance incorporating a metal, including alloys, mixtures and compounds.
The term xe2x80x9coxidizerxe2x80x9d refers to a substance that readily yields oxygen or other oxidizing substances to stimulate the combustion of a fuel, e.g., an oxidizable metal. Specifically, an oxidizer is a substance that supports the combustion of a fuel or propellant.
The term xe2x80x9cfuelxe2x80x9d refers to a substance capable of undergoing a oxidation reaction with an oxidizer.
The term xe2x80x9cpropellantxe2x80x9d refers to a composition comprising at least one fuel and at least one oxidizer. Other materials may be present, including additives and catalysts. The redox reaction between the fuel and oxidizer provides energy, frequently in the form of evolved gas, which is useful in providing an impulse to move a projectile such as a rocket or spacecraft.
The term xe2x80x9cmatrixxe2x80x9d refers to the solid state of the oxidizer wherein one or more metallic fuel particles are substantially encapsulated or embedded within the solid structure, much like the holes in a piece of foam. The structure of the fuel/oxidizer matrix preferably simulates, maintains, or approximates the molecular order as is found in a solution of oxidizer and fuel particles, albeit with some or all of the solvent molecules removed. As such, in preferred embodiments, the metallic fuel particles are generally uniformly distributed throughout the matrix of solid oxidizer.
The phrase xe2x80x9cintimate mixture,xe2x80x9d as it is used herein, means a mixture in which the components are present in a structure that is not composed of discrete, separate particles of the both materials, instead discrete particles of one component (the metallic fuel) is embedded within a network, crystal, semi-crystalline, amorphous or other solid structure of the other component (the oxidizer) such that the two components cannot be unmixed at the particle level by general physical methods, i.e. one would have to resolvate or disperse the oxidizer in a solvent to unmix.
The term xe2x80x9cPropulsion Potentialxe2x80x9d refers to the Isp (total impulse divided by the weight of propellant) as measured at low, near ambient pressures. This term is used to distinguish these low pressure tests and results from the industry standard measurement and reporting practices, which are generally conducted at very high (1000 psi) pressures.
The following section provides a detailed description of preferred embodiments of the invention. Preferred compositions in accordance with the present invention comprise a metallic fuel component and a solid oxidizer component. These components are combined to form a homogeneous mixture through the utilization of freeze drying and spray drying techniques. Such mixtures show superior burn rate characteristics when compared to prior art fuel-oxidizer mixtures.
The Metallic Fuel
The present invention utilizes a metallic particulate component as the fuel. This component can comprise metals such as aluminum, magnesium, zirconium, beryllium, boron and lithium. The metallic component can also comprise a metal hydride, e.g., aluminum hydride or beryllium hydride. Alternatively, mixtures of particles of different kinds of metals could be used. Other possibilities include alloys of two or more metals, or one or more metals in combination with one or more additional substances, e.g., other metal or nonmetal components, aluminum borohydride or lithium borohydride.
In accordance with the present invention, the most preferred metal fuel is aluminum. Aluminum is the most commonly used metal in solid rocket propellants, and is often selected because it is relatively inexpensive, non-toxic, has a high energy content, and exhibits good burning characteristics. Other preferred metal fuels include metals such as boron, beryllium, lithium, zirconium, sodium, potassium, magnesium, calcium, and bismuth. Mixtures and/or alloys comprising these materials are also contemplated for use in the present invention.
While there are many factors surrounding the use of a particular metal fuel, a primary factor is the ability to get the metal to rapidly chemically react, i.e., combust, and to sustain that chemical reaction. The method of one preferred embodiment enables the formation of an intimate, homogeneous mixture of fuel with oxidizer not possible in prior art methods. The nature of the mixture of oxidizer and fuel in this embodiment may also allow for compositions using fuels that are of lower atomic weight than aluminum to achieve a burn process and burn rate within a preferred range for propellants. Table 1 shows the atomic weights of various potential fuels.
The lower atomic number fuels are desirable in that they have the potential to lower the weight of the motor relative to that for aluminum-based motors. One possible key to the success of such fuels is the existence of an appropriate passivation layer around the metallic particle. That passivation layer exists with aluminum in the form of Al2O3. The Al2O3 layer maintains the stability of the energetic aluminum particle while it is in intimate contact with the ammonium perchlorate oxidizer. If the reaction kinetics are too slow for these fuels when micron-sized particles are used, then nanometer-scale powders can be utilized.
The metallic particles of one preferred embodiment may be prepared by methods known in the art. Micron-sized metallic particles may be formed by methods involving mechanical comminution, e.g., milling, grinding, crushing. Such micron sized particles are commercially available from several sources, including Valimet of Stockton, Calif., and are relatively inexpensive.
Because the burn rate for a mixture of metallic fuel particles and oxidizer particles is dependent in part on average particle size, if a faster burn rate is desired, for some embodiments of the present invention it may be advantageous to use particles smaller than micron sized metallic particles produced by mechanical comminution. Nanometer-scale particles may be prepared by either the gas condensation method or the ALEX (exploded aluminum) method. In the gas condensation method, aluminum metal is heated to a vapor. The vapor then collects and condenses into particles. The particles thus produced are nominally spherical, approximately 40 nm in diameter and have a very tight size distribution (xc2x15 nm to 10 nm). These particles are single crystals with negligible structural defect density and are surrounded by an aluminum oxide passivation layer approximately 2.5 nanometers in thickness.
In the ALEX method, a fine aluminum wire is placed in a low pressure inert gas and an electrical current is applied. The electrical discharge through the wire explodes it into aluminum vapor. The particles thus produced range in size from about 100 nm to 500 nm. Nanoaluminum made by the ALEX process is commercially available from several sources, including Argonide of Pittsburgh, Pa.
The rate of energy release for conventional metal fuels is relatively slow because of the relatively large (micron-sized) particle sizes utilized. Nanometer-sized metal powders demonstrate superior performance in this regard by virtue of their very small particle size. Because of the particles"" very small size, both the thermal capacity of each particle and the distance from the core of the particle to the outer surface area where chemical reactions can take place are greatly reduced. Preferably, the metal fuel particles used in preferred embodiments of compositions and propellants have a diameter of about 10 nanometers to about 40 micrometers, more preferably about 10 nanometers to about 10 microns. In one preferred embodiment, the fuel particles have a diameter of about 0.1 micrometer to 1 micrometer. In other preferred embodiments, the fuel particles have a diameter of about 20 nanometers to about 40 nanometers. Methods of preparing nanometal particles are known in the art (e.g. xe2x80x9cOxidation Behavior of Aluminum Nanoparticlesxe2x80x9d, C. E. Aumann, G. L. Skofronick, and J. A. Martin, J. Vac. Sci. Technol. B 13(3), 1178, (1995); xe2x80x9cUltrafine Metal Particlesxe2x80x9d, C. G. Granqvist and R. A. Buhrman, J. Appl. Phys., 47, 2200, (1976).).
The Solid Oxidizer Matrix
One preferred embodiment utilizes an oxidizer, preferably a solid, which is capable of being dissolved in a solvent. Alternatively, the oxidizer may be one which can be finely dispersed in a solvent or emulsified in a solvent or combination of solvents. One preferred solid oxidizer for use in conventional propellant formulations is ammonium perchlorate (AP). AP is a preferred oxidizer because of its ability to efficiently oxidize aluminum fuel to generate large quantities of gas at high temperature. Ammonium perchlorate is also highly soluble in water, dissolving to form an ionic liquid, making it particularly suitable for use in preferred embodiments.
There are several other preferred oxidizers for use in accordance with one preferred embodiment, including hydroxy ammonium perchlorate (HAP), ammonium nitrate (AN), cyclotetramethylene tetranitramine (HMX), cyclotrimethylene trinitramine (RDX), triaminoguanidine nitrate (TAGN), lithium perchlorate, sodium perchlorate, potassium perchlorate, lithium nitrate, sodium nitrate, and potassium nitrate. Any of these or other oxidizers, or mixtures thereof, may be used in preferred embodiments provided that they are capable of being dissolved, dispersed, suspended, emulsified or otherwise distributed into suitably small portions when placed in a solvent or solvent system such as a mixed solvent or emulsion, which may be polar, nonpolar, organic, aqueous, or some combination thereof. Preferred solvents or solvent systems are selected on the basis of their ability to dissolve, solvate, or disperse the oxidizer, while maintaining a minimum of reactivity towards the metallic fuel and oxidizer, at least for the time needed to complete the reaction. In accordance with a preferred embodiment, water is used as the solvent for AP.
The Metallic Fuel Particlexe2x80x94Solid Oxidizer Mixture
The reaction of AP (chemical formula NH4ClO4) with Al fuel is given by the chemical reaction:
2NH4ClO4+4Alxe2x86x922Al2O3+2HCl+N2+2H2O+H2xcex94Hrxn.2.5 kcal/g
The weight ratio of AP to aluminum for a stoichiometric mixture, i.e., no excess oxidizer or fuel, is 42:19. AP will generally not react with aluminum oxide (Al2O3), favoring reaction with unoxidized aluminum metal, so the passivation layer forming the surface of the aluminum particle must be taken into consideration when calculating the proportions of AP to Al for a more precise stoichiometric mixture. When the aluminum is in the form of micron-sized particles, the Al2O3 passivation layer, which is approximately 2.5 nm thick, is practically negligible in weight compared to that of the unoxidized metallic aluminum within the particle. However, when the aluminum is in the form of nanometer-sized particles, the aluminum oxide passivation layer can comprise a substantial portion of the total weight of the particle, e.g., 30 to 40 wt. % or more. Therefore, when nanometer-sized particles are used, less oxidizer per unit weight aluminum fuel is needed for a stoichiometric mixture.
In order to maximize burn rate, or reaction velocity, it is important that the mixture of the metallic fuel and oxidizer be as homogeneous as possible. This is because the burn rate is determined by the reactant diffusion distance, or how far the reactants must travel in order to react with each other. The shorter the distance, the faster the two components can get together to react. In a well-mixed powder made up of metallic particles and oxidizer particles, the reactant diffusion distance corresponds to average particle size.
Minimizing the reactant diffusion distance using conventional methods of preparing propellants can be difficult. If the metallic fuel particles and oxidizer particles are mechanically mixed into a powder, then in order to minimize reactant diffusion distance, the metallic particles and oxidizer particles should both be as small as possible. Under the current state of the art, nanometer scale metal particles can be prepared. However, the smallest particle sizes that have commonly been achieved for ammonium perchlorate are on the order of a few microns in diameter. Therefore, if nanometer metal particles are used with micron-sized (e.g., 3 xcexcm in diameter) oxidizer particles, reducing the particle size of the metal further will not have an appreciable effect on reactant diffusion distance since the oxidizer particle diameter dominates.
Another problem with achieving homogeneous mixtures via the conventional mechanical mixing techniques is that the metal particles or oxidizer particles can agglomerate, resulting in pockets of metal particles directly in contact with each other rather than the oxidizer, and vice versa. Such agglomeration will also increase the reactant diffusion distance, resulting in a slower burn rate.
A number of approaches for dealing with some of these concerns are disclosed in the prior art. One prior art approach to dealing with particle size utilizes a continuous process for preparing a solid propellant wherein an aqueous saturated solution of an oxidizer is added to an aqueous suspension of metal fuel particles. Particles of oxidizer containing occluded metal particles are then crystallized from solution. The metal particle-containing oxidizer particles are then recovered and the aqueous oxidizer solution is recycled. Another prior art method of tailoring solid rocket propellants involves addition of metal fuel particles to a saturated solution of oxidizer. The oxidizer then crystallizes out of solution, producing a precipitate consisting of metal particles coated with oxidizer. While both of these methods can produce a propellant wherein the metal particles coated with or encased within oxidizer, they have the disadvantage of not allowing the stoichiometry of metal to oxidizer to be accurately controlled.
Preparing the Mixture of Metallic Fuel Particles and Solid Oxidizer
In preferred embodiments, reactant diffusion distance is minimized by dispersing the metal fuel particles generally uniformly throughout a matrix of solid oxidizer. The techniques by which this is attained allow for the control of the average distance separating the components in the resulting composition. The means by which this dispersion of metal fuel particles in a solid oxidizer matrix is prepared in the method of one preferred embodiment involves preparing a solution of the oxidizer and adding the metal particles to the solution. The amount of metal particles relative to the amount of oxidizer in solution is preferably adjusted to provide a substantially stoichiometric mixture of fuel to oxidizer. Alternatively, a non-stoichiometric mixture of fuel to oxidizer may be prepared wherein the ratio of the two components is pre-selected. For solid rocket propellant applications, a substantially stoichiometric mixture is preferred. In the case of AP+Al mixtures, a stoichiometric mixture comprises approximately 31 wt. % Al (unoxidized metal) and 69 wt. % AP. Preferably the amount of aluminum in the unoxidized state varies no more than about 5%, more preferably 2% from the 31% by weight midpoint. In circumstances where a non-stoichiometric mixture is desired, the appropriate quantities of metal fuel component and oxidizer component can be selected to provide the desired ratio of fuel to oxidizer.
If desired, additional components may be added to the solution prior to the solvent removal step. These components may include soluble or insoluble solids, e.g., fuels, oxidizers, additives, emulsifiers, etc. Liquids that are miscible or immiscible in the solvent may also be added. Soluble or insoluble gases may also be introduced into the solution.
Generally the preparation of the compositions of a preferred embodiment proceeded as follows. An oxidizer, such as ammonium perchlorate (e.g., commercially available from Aldrich and Alfa) is dissolved with agitation in water to form a solution. The water used may include deionized water, distilled water, tap water or ultrapure water. The dissolution is preferably conducted at room temperature, although a suitable reduced or elevated temperature may be used. Preferably, approximately 20 parts by weight AP is used per 100 parts by weight water, although other suitable concentrations may be used. The concentration is preferably maintained sufficiently below the supersaturation level so that premature crystallization of the AP does not take place. Any suitable means of mixing the AP and water may be used, including agitation, or mechanical stirring. Metal fuel powder is added to the oxidizer solution thus produced. The quantities of oxidizer and metal fuel are selected so as to yield the desired stoichiometry between the components which is desired in the final composition. Other additional components may be added at any point in the process as desired.
After the metal particles and optional additional components are added to the solution, the insoluble components, including the metal fuel particles, must be generally uniformly distributed throughout the solution. One way in which a generally uniform distribution may be obtained is by agitating the solution, but any other suitable method for obtaining a generally uniform distribution may be utilized. Care must be taken to make sure that the solid particles are not allowed to settle out of solution. Smaller particles will take longer to settle out of solution than larger particles.
Once a generally uniform dispersion of particles throughout the solution is achieved, the next step involves removing the solvent from the mixture while preserving the homogeneous, intimate mix. Any suitable method for removing the solvent may be used. Suitable methods include spray drying and freeze drying.
Spray drying is widely used in industry as a method for the production of dry solids in either powder, granulate or agglomerate form from liquid feedstocks as solutions, emulsions and pumpable suspensions. The apparatus used for spray drying consists of a feed pump, rotary or nozzle atomizer, air heater, air disperser, drying chamber, and systems for exhaust air cleaning and powder recovery. In spray drying, a liquid feedstock is atomized into a spray of droplets and the droplets are contacted with hot air in a drying chamber. Evaporation of moisture from the droplets and formation of dry particles proceed under controlled temperature and airflow conditions. The powder, granulate or agglomerate formed is then discharged from the drying chamber. In some cases, it may be necessary to continue the stirring or agitation of the solution during the spray drying process so that the composition made at the end of the spraying procedure is still well mixed. By adjusting the operating conditions and dryer design, the characteristics of the spray dried product can be determined.
The spray drying method is especially preferred when the contact time between the metal particles and solvent need to be minimized. For example, when nanometer-sized aluminum particles are placed in room temperature water, they will completely react to form Al2O3 in less than 24 hours. Because of the small particle size, the reaction occurs very quickly once the passivation layer is penetrated. By using a spray drying technique, the time in which the aluminum particles are in contact with the water solvent can be minimized.
Another preferred method for removing the solvent is freeze drying. Freeze drying consists of three stages: pre-freezing, primary drying, and secondary drying. Before freeze drying may be initiated, the mixture to be freeze dried must be adequately pre-frozen, i.e., the material is completely frozen so that there are no pockets of unfrozen concentrated solute. In the case of aqueous mixtures of solutes that freeze at lower temperature than the surrounding water, the mixture must be frozen to the eutectic temperature. Once the mixture is adequately pre-frozen, then the solvent is removed from the frozen mixture via sublimation in the primary drying step. After the primary drying step is completed, solvent may still be present in the mixture in bound form. To remove this bound solvent, continued drying is necessary to desorb the solvent from the product.
In accordance with a preferred method of freeze drying used in making the compositions of the present invention, the freeze drying process is preferably initiated by pouring the mixture into a container immersed in a cryogen, such as liquid nitrogen or a dry ice/acetone bath. Similarly, the container in which the mixture was made may be immersed or otherwise exposed to a cryogenic liquid or placed in a freezer. In order to maintain the homogeneity of the mixture, it may be necessary to continue the stirring, agitation or other mixing means during the freezing process. Once the mixture has completely frozen the container of frozen mixture is then transferred to a vacuum container.
Preferred freeze drying apparatuses include standard high-vacuum chambers that are pumped by high-pumping-speed diffusion pumps. Such chambers are available commercially (e.g., the Varian VHS-6 cart-mounted pumping assembly #3307-L5045-303 with a 12xe2x80x3-diameter stainless steel bell jar assembly) and are in common use for vacuum deposition of metallic films and general purpose vacuum processing. An alternative, similar system can be assembled from off-the-shelf vacuum components available from a variety of suppliers. The specifics of the vacuum design are not critical, as long as the design incorporates high pumping speed (preferably 2000 liters/sec or better) and low ultimate pressure. Active pumping on the vacuum container is initiated as soon as practical after freezing the mixture. After a period of about 20 to 60 minutes, depending upon the specific pumping characteristics and volume of the vacuum chamber, the pressure in the system achieves a steady state near the equilibrium vapor pressure of the frozen solvent (in the 10xe2x88x923 Torr range for water). The temperature during the process is preferably xe2x88x9215 to xe2x88x925xc2x0 C., more preferably xe2x88x9210xc2x0 C. when water is used as the solvent. The pressure is maintained at this steady state while the frozen water in the mixture is removed from the mixture by sublimation (i.e., direct conversion of solid to gas). The period of time required to remove water by sublimation depends upon the batch size being processed. As an example, a 0.5 liter volume of frozen mixture containing 50 grams of propellant solute requires approximately 100 hours to remove the water, depending upon the pumping speed of the vacuum system. After removal of the water is complete, as indicated by a rapid drop in the steady-state pressure to a value near the base pressure of the vacuum container (i.e., 10xe2x88x925 Torr or lower), the material consists of low-density, dry agglomerates of a metal fuel particles distributed generally uniformly throughout a matrix of the oxidizer.
Freeze drying techniques have been utilized to facilitate mixing of the solid rocket propellant components. One prior art method concerns a low shear mixing process for preparing rocket propellants. The propellant ingredients are blended with an inert diluent to reduce the high shear mixing environment generated by conventional mixing techniques. Once thus mixed, the diluent is removed by sublimation from the mixture via a freeze drying process. While this method does facilitate the mixing of high solids propellants, the individual components, i.e., the oxidizer and metallic fuel, still comprise discrete particles. Thus, the problems of achieving a homogeneous mixture inherent in mixing discrete oxidizer and metallic particles are still present in this method.
In preferred methods, freeze drying techniques are used to prepare ultrafine particles comprising metallic particles generally uniformly dispersed in a matrix of solid oxidizer, thereby eliminating the problems inherent in the use of discrete metallic fuel particles and solid oxidizer particles. The freeze drying method used in accordance with preferred embodiments involves forming a generally uniform dispersion of metal particles in the solution of solid oxidizer. Water is a preferred solvent because it will dissolve a wide range of solid oxidizers, many of which are ionic solids. Of the ionic solid oxidizers, ammonium perchlorate is preferred because of its good solubility in water.
Once the solution is prepared and the solid particles are generally uniformly dispersed in solution, it is rapidly cooled to freeze the solution and fix the spatial distribution of particles throughout the solution. Any suitable cooling and freezing method may be used, but preferred methods involve immersing the solution in a cryogenic liquid, e.g., liquid nitrogen. The frozen liquid is then transferred to a vacuum chamber where solvent is removed by sublimation. This method works well with nanoaluminum since the metal is sufficiently non-reactive at cryogenic temperatures. In addition, the method is particularly well suited for use with nanoaluminum since nanometer-sized particles remain suspended in the solvent for a period of time than do micrometer-sized particles. This feature enables the nanoaluminum mixture to be rapidly frozen without undue settling of the aluminum particles to the bottom of the freezing volume, with little or no agitation required during freezing. Nanometer-sized particles form a pseudo-colloidal suspension with the solvent, whereas micron-sized particles rapidly settle out of the mixture unless continuous agitation is applied during freezing.